1. Field of the Invention
This invention relates to ring laser gyroscopes; and, in particular, to four mode multi-oscillator ring laser gyroscopes having a non-reciprocal polarization rotator which incorporates a very uniform, low gradient magnetic field.
2. Description of the Related Art
Since its introduction in the early 1960s, the ring laser gyroscope has been developed as a logical replacement for the mechanical gyroscope for use in all manner of inertial guidance systems. Heretofore, the basic two mode ring laser gyroscope has been developed which has two independent electromagnetic waves oscillating in an optical ring cavity. When the ring is stationary, no rotation is indicated. As the ring cavity is rotated about its central axis, the counter-rotating waves interact with one another so that a beat frequency is developed. A linear relationship between the beat frequency and the rotation rate of the cavity with respect to the inertial frame of reference may be established. Ideally, the rotation rate is proportional to the beat note. In this manner, a gyroscope is theoretically produced having no moving parts. In practice, however, the two mode laser gyro often must be mechanically dithered to keep the counter-rotating traveling waves from locking at low rotation rates.
In an effort to solve this lock-in problem, non-planar ring cavities have been designed containing more than one pair of counter-rotating modes. These multi-oscillator ring laser gyros have been developed to achieve the goal of an accurate gyroscope having no moving parts. Even the multi-oscillator ring laser gyro requires the use of a non-reciprocal polarization rotation device (such as a Faraday rotator) to achieve the splitting of the light within the ring cavity into two pair of counter-rotating modes. Generally, the multi-oscillator ring laser gyro is divided into a pair of right circularly polarized and left circularly polarized waves. The right circular polarized waves are split by the Faraday rotator into clockwise and anticlockwise modes. Likewise, the left circularly polarized waves are split by the rotator into clockwise and anticlockwise modes.
The ring laser gyroscope is an active device; it sustains laser light by providing its own optical gain over a small range of optical frequencies. The gain profile can be considered to be a bell-shaped distribution of optical amplification over this range of optical frequencies. The standard spectrum of laser light in a multi-oscillator ring laser gyro contains right and left circularly polarized modes. Two laser modes, of opposite circular polarity, are established on opposite sides of the center frequency of the cavity's gain profile.
Each of these two circularly polarized light beams is separated into two counterpropagating beams, at frequencies distinct from one another, by the non-reciprocal polarization rotation provided by the Faraday rotator assembly. These four beams, then, constitute the four mode spectrum of the multi-oscillator laser gyro.
This non-reciprocal polarization rotation induces for the two circular polarizations an equal and opposite frequency splitting between counterpropagating modes. This is to say, a beam of one circular polarization ends up with its clockwise propagating mode at a higher frequency than its counterclockwise propagating mode, and light of the other polarization has its counterclockwise mode at a higher frequency that its clockwise mode. The frequency separation between the counterpropagating modes is, ideally, equal for the two circular polarizations.
Since a rotation of the cavity will cause opposite frequency shifts in the clockwise and counterclockwise propagating modes, any rotation of the gyroscope will cause the frequency separation between counterpropagating modes to increase for light of one circular polarization, while the frequency separation between counterpropagating modes decreases for light of the other circular polarization. It is this difference in frequency separations of counterpropagating modes which is, ideally, proportional to the rotation rate of the cavity.
The exact frequencies, .lambda., of these modes are determined by the apparent optical path length, L, of the cavity, through L=N.lambda. for some integer N. The apparent optical path length is, in turn, effected by the apparent index of refraction of the gain medium. The apparent indices of refraction seen by the four modes are different from one another, and depend on the optical gain available at a particular frequency and on the optical loss experienced by the lasing mode. If the gain or loss of a laser mode is altered thereby changing the apparent index of refraction and optical path length in the gain medium, the lasing frequency of the mode will adjust itself to remain consistent with the L=N.lambda. relationship.
Differences in optical losses among the four beams in the cavity can affect the frequency separations between counterpropagating beams. Non-uniform magnetic fields in the Faraday rotator assembly have been identified as a source of differences in optical loss among the laser modes which cause a difference in the frequency separations of counterpropagating modes of the two circular polarizations. This effect is indistinguishable from the effects of rotation of the cavity.
One can measure the difference in frequency separations of counterpropagating beams while the gyroscope is subject to rotation of a known angular rate. A difference in frequency separations of counterpropagating modes that is not due to the known rotation rate can be taken as being the bias of the gyroscope. Unfortunately, the bias due to non-uniform magnetic fields heretofore used in the Faraday rotator assembly changes as the temperature of the gyroscope changes. In fact, these non-uniform magnetic fields have been identified as being among the largest sources of temperature sensitivity of multi-oscillator ring laser gyroscope bias.
In actual operation of the multi-oscillator ring laser gyro, however, the bias is not stable over all operating conditions. For example, over a wide temperature range thermal bias sensitivity is exhibited due to the mode pulling effects of the media, as well as the Faraday rotator, as it acts to split each set of circularly polarized light into its respective clockwise and anticlockwise modes.
What is needed is a non-reciprocal polarization rotation device which may be used to split each set of right and left circularly polarized waves into respective clockwise and anticlockwise modes without causing undue differential mode pulling. Heretofore, when a Faraday rotator has been used to perform a non-reciprocal polarization rotation of the propagating waves within the ring laser cavity, the helicity factor of the Faraday rotator causes opposite concave and convex lensing effect depending upon the helicity of the light which approaches the Faraday rotator glass. Over a wide temperature range, this helicity effect changes so that the mode pulling that occurs is not uniform and may not be factored out, as a constant bias, in any rotation rate calculation.